Recently, with the invention of electric vehicles and hybrid vehicles, the importance of a super-capacitor, which is used as a power supply device together with a lithium secondary cell, has been emphasized. Thus, researches are being actively conducted to develop a super-capacitor, which is small and light and has a high-output and high-capacity performance. Materials that have been researched for a super-capacitor electrode include carbons, metal oxides, conductive polymers, and others. Especially, the metal oxide is advantageous because it exhibits larger specific capacitance than that of other electrode materials by virtue of the additional Faraday reaction between the electrode material and an electrolyte. Among the metal oxides, a ruthenium oxide was the first material that had been researched as an electrode material over a long period of time. The ruthenium oxide has recorded significantly high specific capacitance over other metal oxides. However, due to a small preservation amount of a precursor and a high price, there have been demands for a research of development of an alternative metal oxide.
Especially, a manganese oxide has drawn attentions as an electrode material, which can satisfy the high specific capacitance of the ruthenium oxide and substitute the ruthenium oxide by virtue of a large preservation amount of a precursor, a low price, and easiness in synthesis. In spite of these advantages, however, since the manganese oxide has low electrical conductivity, there is limitation in actually utilizing the manganese oxide. In order to overcome the limitation, many researchers have focused on improving the characteristic of the manganese oxide as an electrode material by mixing the manganese oxide with a material having high electrical conductivity. Among various possible materials, graphene is a material formed of one carbon atomic layer and having a two-dimensional plane structure. Such graphene has many excellent physical properties including high electrical conductivity. Thus, in addition to researches on the graphene material itself, many researches on use of the graphene as an electrode material by mixing the graphene with a transition metal oxide including a manganese oxide are being conducted.
With respect to one of the most critical factors in selecting an electrode material forming an electrode, an electrode material should have many active surfaces exposed by a large specific surface area. This factor is related to a diffusion velocity of ions of an electrolyte and expansion of an accessible surface. Eventually, the factor is related to high specific capacitance. Accordingly, many researches are being conducted as to evaluation of applicability of a metal oxide in a 1D or 2D structure having a much large surface area compared to a bulk phase as an electrode material for the capacitor.
Other several researches have reported methods for synthesizing the manganese oxide and a nanocomposite. Most of the synthesis methods being currently used relate to supply an external power by using an electrodeposition method, hydrothermal synthesis, or others (Korean Patent Application Publication No. 10-2010-0037689) or require a condition of high temperature and high pressure and a special equipment. Thus, the synthesis methods are vulnerable in the aspects of energy and economy.